1. Field of the Invention
This invention relates to systems and methods for using Fourier transform optics to inspect devices.
2. Description of Related Art
Inspecting substrates used in integrated circuits or flat panel displays typically relies on one of three methods: scanning the substrate with a beam that generates a signal indicating reflection, transmission, or absorption of the beam in a small portions of the substrate; imaging the substrate and analyzing the resulting image; or forming the Fourier transform of a light pattern from the substrate and analyzing the Fourier transform. For these methods, reducing the area simultaneously inspected can improve the resolution (i.e., detect smaller defects or features). For scanning, a smaller beam diameter isolates the attributes of a smaller portion of the substrate. For imaging, higher magnification increases the size of features in the image but decreases the substrate area inspected by a fixed size detector. For Fourier transform inspections, transforming a light pattern from a smaller area of the substrate reduces the area represented by the Fourier transform but makes light from a small feature a larger portion of the light in the Fourier transform and therefore more easily detectable. In all these cases, decreasing area simultaneously inspected improves resolution but decreases the speed of inspection by requiring more scans, more images, or more exposures to cover a substrate.
Fourier transform based inspection systems which scan a substrate can provide fast inspection because Fourier transform optics perform "optical computing" which separates a defect signal from a non-defect signal. Optical computing allows the area illuminated by a scan beam to be large when compared to beams used in a conventional scanning system. For example, a conventional scanning system for a substrate having a feature size of a few microns might use a beam which is a few microns in diameter. A Fourier transform inspection system for the same substrate would use a beam which is millimeters in diameter. The large beam for the Fourier transform optics encompasses repetitive features which create a characteristic spatial frequency light pattern that is transformed to a distinctive Fourier transform pattern. However, when inspecting for defects about a micron in size, the amount of light reaching the Fourier plane from a defect is typically small relative to the total amount of light reaching the Fourier plane. The small signal generated by the defect may be of the same order of magnitude as the noise associated with the non-defective signal, making defect detection difficult.
Reducing the area of illumination improves the signal-to-noise ratio by making light from a defect a larger fraction of the total light, but reducing the illuminated area also increases the number of scans required to cover the substrate. For example, changing the beam diameter from 0.15 mm to 2 mm changes the inspected area by more than a factor of fifty, and requires increasing a scan rate by a similar factor to maintain the original inspection time. This magnitude of change in scan rate is typically not achievable.